1. Field of the Invention
The present invention relates to a liquid crystal display device including liquid crystal elements in which a liquid crystal driving circuit is improved.
2. Description of the Related Art
In a normal active matrix type liquid crystal display device, a scanning period of one screen (one frame) is approximately 50 Hz to 75 Hz (13.3 msec to 20 msec). On the other hand, an optical response of a liquid crystal molecule requires a time of several tens msec. Thus, in the case where moving pictures such as TV are displayed on the liquid crystal display device, the response of the liquid crystal element can not follow the change of display data and there arises a disadvantage that a residual image is produced.
Conventionally, as one of measures to the residual image, a measure in which attention is paid to the application voltage dependency of the response speed of the liquid crystal molecule has been taken.
FIG. 4 is a schematic view showing a relation between a liquid crystal application voltage (signal voltage) and a liquid crystal response (luminance change) in a conventional liquid crystal display device of a normally white mode.
In FIG. 4, reference numeral 1 designates a liquid crystal application voltage (signal voltage) when the change of the application voltage is small; 2, a liquid crystal application voltage (signal voltage) when the change of the application voltage is large; 3, a luminance change when the liquid crystal application voltage (signal voltage) 1 is applied; and 4, a luminance change of the liquid crystal element when the liquid crystal application voltage (signal voltage) 2 is applied.
FIG. 5 is a schematic view showing a relation between a liquid crystal application voltage and a response of the liquid crystal element (luminance change) using a conventional Overdrive Method.
In FIG. 5, reference numerals 1 and 3 designates the same as those in FIG. 4. Reference numeral 5 designates a liquid crystal application voltage (correction voltage) applied prior to the liquid crystal application voltage (signal voltage) 1 in order to speed up the response of the liquid crystal element to the liquid crystal application voltage (signal voltage) 1; and 6, a luminance change responding to the liquid crystal application voltage (correction signal) 5.
FIG. 6 is a view showing a gray level-luminance characteristic of a conventional liquid crystal display device of 8-bit (256 levels) display.
In FIG. 6, reference numeral 7 designates a gray level-luminance characteristic.
FIG. 7 is a view showing a liquid crystal application voltage-luminance characteristic of a conventional liquid crystal display device of 8-bit (256 levels) display.
In FIG. 7, reference numeral 8 designates the liquid crystal application voltage-luminance characteristic. Besides, symbol NUR designates a normal use range.
Next, an operation will be described.
FIG. 4 is based on the liquid crystal display device of the normally white mode in which a white display is carried out in a state where an effective voltage is not applied to the liquid crystal element. As shown in FIG. 4, when the liquid crystal application voltage (signal voltage) 1 or 2 is changed, the liquid crystal element starts to respond as indicated by the luminance change 3 or 4, and like the liquid crystal application voltage (signal voltage) 2 and the luminance change 4, the larger the amount of change of the liquid crystal application voltage (signal voltage) is, the shorter the time until the response is completed is. That is, the response of the liquid crystal element between white and black is quick as compared with the response of the liquid crystal element between gray levels. Then, as shown in FIG. 5, in the case where a dark gray level is changed to a bright gray level, a voltage lower than a steady voltage after the change is temporarily applied like the liquid crystal application voltage (correction voltage) 5, and the optical response of the liquid crystal element is made quick like the luminance change 6. In the case where a bright gray level is changed to a dark gray level, a voltage higher than a steady voltage after the change is temporarily applied to speed up the optical response of the liquid crystal element. The liquid crystal response property between gray levels can be improved by correcting the liquid crystal application voltage like this.
In order to realize 8-bit (256 levels) multi-gray levels as shown in FIG. 6, positive and negative reference voltages of approximately 10 to 18 levels in total are normally inputted to a liquid crystal driving circuit, voltages between the respective reference voltages are divided by the liquid crystal driving circuit on the basis of the reference voltages, output voltages of 256 levels are generated in the respective polarities, and an output voltage corresponding to inputted data is selected and is outputted.
Reference symbols V0 to V17 of the liquid crystal application voltage-luminance characteristic 8 of FIG. 7 designate reference voltages inputted to the liquid crystal driving circuit in order to realize the gray level-luminance characteristic 7 of FIG. 6. Among these reference voltages, V8(P)/V9(N) corresponding to a white display is set to a voltage at which the relative luminance becomes approximately 100%, and V0(P)/V17(N) corresponding to a black display is set to a voltage at which a sufficient contrast ratio can be obtained. Here, characters (P) and (N) mean (Positive) and (Negative), and express a positive reference voltage and a negative reference voltage, respectively.
In the conventional reference voltage setting like this, in the case where a gray level is changed from a bright gray level to a gray level close to a saturated gray level (hereinafter referred to as “white”, 255 level in eight bits), a voltage value which can be selected as a correction value of the liquid crystal application voltage is a white level one at the minimum, and there is a gray level in which the correction voltage is insufficient so that the speed of a liquid crystal response property can not be made high. Also in the case where a gray level is changed from a dark gray level to a gray level close to 0 level (hereinafter referred to as “black”), since a voltage value which can be selected as a correction value of the liquid crystal application voltage is a black level one at the maximum, a similar problem arises.